I don't know the exact charging structure at Biggin, but it is usual for home based aircraft to not be charged for circuits. So if the circuit is full of 3 or 4 GA types all doing Touch and Goes, then that makes about 40 movements per hour (almost as many as Heathrow.), and the airfield will not be making one penny. Remember that for each circuit there will be at least two 'interactions' with ATC, so they will be extremely busy, and hardly have time to draw breath.
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